Foreign Secretary Liz Truss has expressed her support to Brits heading to Ukraine to fight against the Russian invasion.
Ukraine President Volodymyr Zelensky promised to arm foreign volunteers who to travel to his country to join the battle against Putin’s forces.
The foreign minister of Ukraine, Dmytro Kuleba, echoed his calls.
"Foreigners willing to defend Ukraine and world order as part of the International Legion of Territorial Defence of Ukraine, I invite you to contact foreign diplomatic missions of Ukraine in your respective countries.
"Together we defeated Hitler, and we will defeat Putin, too."
Asked if she would support UK citizens who chose to answer the call, Ms Truss told the BBC : “That is something people can make their own decisions about.
“The people of Ukraine are fighting for freedom and democracy, not just for Ukraine but for the whole of Europe.
“Absolutely, if people want to support that struggle I would support them in doing that.”
Western officials have said Putin's invasion is not going according to plan as Ukrainians are showing strong resistance.
But those officials raised the prospect that the Russian could use thermobaric “vacuum bombs” which suck in oxygen to create a devastating, high temperature blast, if Putin becomes desperate.
Ms Truss said the survival of Mr Putin’s regime could now be at stake if his invasion plan failed.
“This could well be the beginning of the end for Putin. I fear that he is prepared to use the most unsavoury means in this war,” she told Sky News’s Trevor Phillips on Sunday programme.
She also warned of a conflict between Russia and Nato if Putin is not stopped in Ukraine.
Vowing to step up sanctions, she told the BBC: "This long-running conflict is about freedom and democracy in Europe.
“If we don’t stop Putin in Ukraine we are going to see others under threat – the Baltics, Poland, Moldova, and it could end up in a conflict with Nato."
The UK's Chief of Defence Intelligence Sir Jim Hockenhull said Russian forces were targeting the Ukrainian capital in a bid to install a pro-Moscow government.
Tory MP Tobias Ellwood, chair of the Commons Defence Select Committee said Britain's options are limited with direct conflict off the table.
But he called on leaders to launch sanctions every day, including booting Russia's ambassador to the UK from the country.
